Laser printers of the electrophotographic type typically use a laser beam scanning optical apparatus having a laser diode as a light source to write images on the surface of a photosensitive member. A laser beam emitted from the laser diode is condensed by a collimator lens, deflected at equiangularity by a deflection device, and scans the surface of a photosensitive member at uniform speed via an f.theta. optical element.
In laser beam scanning optical apparatuses of this type, there is a tendency of changes in the oscillation wavelength of the laser diode, changes of the refractive index and shape of the lenses, and changes of the spacing between lenses to occur in conjunction with fluctuations of environmental temperature, which cause fluctuation of the focal length of the optical system and result in disadvantages including fluctuating beam spot diameter on the image plane (i.e., photosensitivemember), and poor image quality. Variation of the condensed state of the laser beam occurring due to fluctuation of the environmental temperature is particularly marked when the lens is formed of resin materials.
In order to counteract the aforesaid disadvantages, Japanese Laid-Open Patent Application No. 2-58016 discloses a method for correcting the beam convergence position by optically detecting the state of convergence of the laser beam at equivalent positions on the photosensitive member and moving the focusing lens to make said correction. In optical devices of this type, however, lens systems have different optical powers in the main scan direction and the sub-scan direction, such that when, for example, the convergence position is adjusted only in the main scan direction, such adjustment is not attained in the sub-scan direction.
Japanese Laid-Open Patent Application No. 4-58211 proposes a method of separate focus adjustments for the main scan direction and the sub-scan direction. Separately adjusting the focus in the main scan and sub-scan directions, however, increases the size, complexity and cost of the adjustment mechanism.